Beit Lahiya, Gaza Strip ? A large roadside bomb exploded Wednesday beneath an American diplomatic convoy, killing three security officers in the first deadly attack on a U.S. target since the Palestinian uprising began three years ago.

The blast upended and nearly sliced an armored Chevrolet Suburban in half, spraying car and body parts and gouging a large crater in the road barely a mile from the checkpoint where the convoy had entered Gaza from Israel. One person in the blown-out car was wounded but survived.

The passengers of two other U.S. vehicles escaped unharmed.

The three security officers who died in the attack were escorting cultural attaches sent to interview Palestinian academics who had applied for Fulbright scholarships to study or teach in the United States.

The attack raised the possibility of harsh U.S. and Israeli reprisals, although knowing exactly whom to punish may prove difficult. All of the major Palestinian armed factions, such as Hamas and Islamic Jihad, denied involvement. These groups have not targeted Americans in the past.

Still, there was an immediate sense that a new line had been crossed in the Palestinian intifada, pushing it into a wider arena that now puts American lives in jeopardy in retribution for Washington’s unwavering support of Israel.

“It’s definitely a watershed event,” said Shmuel Bar, an Israeli intelligence veteran and expert in radical Islam. “You have to feel they were targeted. You don’t mistake them for Israelis.”

After the bombing, the U.S. Embassy in Tel Aviv urged all American citizens to leave Gaza, a stronghold of Islamic extremist groups — such as Hamas and Islamic Jihad. Acting on a request from Palestinian leaders, the FBI dispatched a team of agents from Washington to help with the investigation.

Palestinian investigators inspect the wreckage of a U.S. Embassy vehicle near the village of Beit Lahiya in the northern Gaza Strip. A massive explosion Wednesday ripped through the U.S. diplomatic vehicle, killing three Americans. The bombing marked the first time in the three-year Palestinian uprising that Americans were targeted for violence.

Israeli terrorism experts described the attack as well-organized, possibly with inside knowledge of when the Americans would be driving into Gaza on Wednesday morning in their fortified cars. Such delegations are frequent visitors here, and their diplomatic license plates and the make of car render them instantly recognizable.

The dead and injured, all Americans, were private security guards from a Virginia-based company employed by the U.S. Embassy to protect its diplomats on visits to Gaza.

The State Department identified the slain Americans as John Branchizio, 36; Mark T. Parson, 31; and John Martin Linde Jr., 30. The wounded American initially was treated at a Gaza hospital before being transferred to a hospital in the southern Israeli city of Beersheba.

Condemnation of the bombing poured in from all sides — the White House, Israeli officials and the flailing Palestinian Authority, which is struggling to form a new government to reverse the deepening sense of chaos in the Palestinian territories and misery under Israeli occupation.

“The explosion was against the peace process and against Palestinian national interests,” said Saeb Erekat, a senior negotiator close to Yasser Arafat, the Palestinian leader.

In Washington, a statement by President Bush blamed Palestinian officials for failing to act against militant groups and singled out Arafat as the main obstacle to that effort.

“Palestinian authorities should have acted long ago to fight terror in all its forms,” Bush said. “There must be an empowered prime minister who controls all Palestinian security forces, reforms that continue to be blocked by Yasser Arafat.”

Palestinian officials, who appeared embarrassed and upset by the bombing, pledged full cooperation with the investigation.

A Palestinian police officer fires into the air as he tries to disperse a violent crowd after a U.S Embassy vehicle was bombed in the Gaza Strip. The massive explosion Wednesday killed three Americans, Palestinian officials said.